The present invention relates to a gas detecting sensor, particularly to a sensor for detecting the partial pressure of oxygen gas contained within exhaust gases discharged from an internal combustion engine of an automobile or the like and measuring air to fuel ratio of the combustion mixture to be supplied into the internal combustion engine.
Recently, "lean burn system" that ia, the system of operating an internal combustion engine with an air to fuel ratio larger than the stoichiometric value thereof so as to reduce harmful components contained within the exhaust gases and lower the fuel consumption, has been proposed and employed practically.
The above described "lean burn system" requires a detecting means for accurately detecting air to fuel ratio in a range of lean mixtures.
U.S. Pat. Nos. 3,933,028 and 4,012,709 disclose examples of such a detecting sensor as described above.
These detecting sensors are provided with a sensing element formed of a solid solution of an alloy of cobalt monoxide(CoO) and magnesium monoxide(MgO) so as to avoid a phase change of cobalt monoxide into tricobalt tetroxide (Co.sub.3 O.sub.4).
The sensing element is heated and maintained at a predetermined temperature, such as 900.degree. C. so as to compensate output fluctuation caused by the temperature change and to prevent the above described phase change, when the detecting sensor is used.
However, the above described sensing element formed of cobalt monoxide-magnesium monoxide has such a problem that the electrical resistance of the element does not largely change in response to the change of the partial pressure of oxygen so that the correct air to fuel ratio cannot be measured.
Furthermore, in the conventional detecting sensor, the heating means has been produced by winding a linear heating member like a coil so that the structure thereof is complex and that the process of mounting the heating means to the detecting sensor is complicated.
And since the sensing element of the conventional detecting sensor has a cylindrical or disc-like shape, the thickness thereof is large so that the element does not rapidly respond to the change of the partial pressure of oxygen.
Since the heating member is provided in such a manner as to surround the sensing element without contacting therewith, it takes a while for heating the sensing element to a predetermined temperature and the consumption of electric power becomes large.
Accordingly, one object of the present invention is to provide a high sensitive sensing element of which electrical resistance largely changes in response to the change of the partial pressure of oxygen gas.
Another object of the present invention is to provide a gas detecting sensor having a simple structure, which comprises a sensing element having excellent responsive characteristic to the change of the partial pressure of oxygen gas, which does not consume large electric power for heating the sensing element and which can be easily produced.